Thin film deposition encompasses a group of techniques for forming a thin layer (i.e. less than 5 .mu.m in thickness) of a solid material on the surface of a workpiece. It is generally desirable that such a film be homogeneous, uniform in thickness, and extremely pure. In the semiconductor industry, for example, the fabrication of microelectronic circuits on a substrate of single-crystal silicon typically requires that several thin films of both insulating and conducting materials be formed and patterned sequentially on the substrate surface. Common thin film deposition techniques include physical vapor deposition such as sputtering, drying a film from a liquid coating deposited by a spin-on technique, and chemical vapor deposition.
Chemical vapor deposition in general involves delivery of one or more constituents or precursors in a vapor phase which, upon contacting a (usually heated) substrate, decompose to produce a film in a surface-catalyzed reaction. Various CVD techniques are commonly used for deposition of epitaxial silicon, polysilicon, silicon dioxide, and silicon nitride. More recently, CVD of tungsten and titanium nitride has been developed. However, CVD of most metal oxides, and particularly of compound metal oxides, has been far less successful, partially because of the unsuitability of available precursors. Known metal precursors often have: low volatility which severely limits achievable deposition rate; limited stability and therefore decompose or react before reaching the substrate; or have ligands (molecules, ions, or atoms attached to a central atom) which fail to completely dissociate from the metal upon reaching the substrate, leaving impurities such as carbon or fluorine in the deposited film.
One promising class of precursors for use with barium, strontium, and calcium, is suggested in U.S. Pat. 5,280,012, issued to Kirlin et al. on Jan. 18, 1994. These metalorganic precursors are of a formula MA.sub.2 X, where M is barium, strontium, or calcium, A is a monodentate or multidentate (multidentate infers a capability to donate two or more pairs of electrons in a complexation reaction) organic ligand coordinated to M which allows complexing of MA.sub.2 with a ligand X to form a stable subcomplex, and X is a monodentate or multidentate organic ligand coordinated to M and containing only atoms selected from the group consisting of C, N, H, S, O, and F.
Unfortunately, additional difficulties arise when thin films of, e.g., compound metal oxides are desired, even if stable, volatile precursors can be found. In the approach used in the '012 Patent for forming such films, separate precursor sources are used for each metal constituent of the film. However, film stoichiometry is difficult to maintain when two or more precursors which react individually on the growing film are used, due to differences in precursor vapor pressures, surface reaction rates, etc. To avoid such stoichiometry problems, single-source precursors have traditionally been sought which already contain a built-in stoichiometry: typically, such precursors contain two desired metallic elements bonded together, with several ligands attached to each element. Upon reaching the substrate, the single-source precursor ideally loses its ligands without changing the metal atom ratios. Unfortunately, single-source precursors have their own set of problems: they often have vapor pressures even lower than those of precursors containing only one metal atom; precursor stoichiometry may still not remain in the film (e.g. the heated substrate may evaporate one element at a significantly higher rate); such precursors are not directly applicable to non-integral stoichiometries; and such precursors are commercially uncommon.
Tightly controlled stoichiometry and grain size, homogeneity, and purity are particularly important for deposition of thin films of polycrystalline perovskite-phase niobates, zirconates, or titanates, such as CaTiO.sub.3 (calcium titanate), BaTiO.sub.3 (barium titanate), SrTiO.sub.3 (strontium titanate), (Ba,Sr)TiO.sub.3 (barium strontium titanate, or BST), or bismuth titanate. Under proper conditions, these materials may possess one or more interesting and useful properties, including: extremely high dielectric constant (generally greater than 200), high remanent polarization (or ferroelectric effect), and a high dependence of dielectric constant on temperature (or pyroelectric effect). As such, perovskites find application in dynamic random-access memory (DRAM), ferroelectric random-access memory (FRAM), infrared detectors, and other circuitry requiting high performance miniature capacitors. A CVD technique useful for such films and applications is highly desirable.